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Statement of Significance (as of designation - December 20, 1989):Constructed by the State of Illinois in 1896, the University of Illinois Observatory is significant for its association with the development of the selenium photoelectric cell, which revolutionized the science of astronomical photoelectric photometry (the measurement of celestial magnitudes). This cell was developed by Joel Stebbins (1878-1966), who served as director of the observatory from 1907 to 1922. As a result of Stebbins' work, photoelectric photometry became the standard technique in determining stellar magnitudes.